That’s not to say we’ll never find evidence of life, but rather that there’s a long, difficult road between suggestive evidence and established fact. We’ve been collecting and critically examining the former for more than four centuries, and we’ve still never reached the latter. In 1610, less than a month after Galileo published the first telescopic observations of the moon, the great astronomer Johannes Kepler declared that they provided evidence of life. Since then, as new instrumentation and methods have allowed us to look further and more clearly into space, scientists have repeatedly sought—and found—evidence of extraterrestrial life. In 1638, Royal Society founder John Wilkins drew on the best scientific data available to agree that the moon likely hosted life. Not until the late 19th century did most practicing astronomers, based upon slowly improved data coming from slowly improving telescopes, become sure that Kepler and Wilkins were wrong.

At the same time, though, new observations from telescopes aimed at Mars began to suggest the red planet hosted a network of canals. In 1894, American astronomer Percival Lowell claimed these must have been built by intelligent life. Not until 1909 did most of the scientific community turn to alternative explanations, and not until 1975 were we sure that Lowell’s canals were tricks of the eye.

Most eureka moments only look like discrete “discoveries” through the simplified hindsight ofhistory.

Yet new data and interpretations kept pouring in. In 1924, new methods for measuring extraterrestrial temperatures and atmospheric compositions suggested temperatures above freezing and atmospheric water vapor on Mars, prompting American physicist William W.Coblentz to suggest that the planet was home to vegetation. Later observation revealed that the detected water vapor rested in Earth’s atmosphere, but only barren surface images taken by 1976’s Viking landers finally refuted the plant hypothesis.

Even so, Viking provided a whole new set of data, some of which could be seen as suggesting the presence of microbial life. Scientific consensus slowly accrued against this interpretation. But 20 years later, microscopic and chemical analysis of Martian meteorites revealed potentially organic formations and life-born molecules. This hypothesis still isn’t dead, and scientists continue to search for other possible signs of life.

This is how science works. Even seemingly revolutionary discoveries—like Copernicus’s recognition that the Earth orbits the sun, or Darwin’s that life evolves through natural selection—were not instantaneous eureka moments. They were key contributions to centuries-long, relatively undramatic processes of observation and explanatory argument. These processes build, step by step, toward answers—answers that only look like discrete “discoveries” through the simplified hindsight of history.

Indeed, the first step in the search for aliens was only recently resolved, after a very long debate. We’ve been sure that many stars besides our Sun host planets for less than 30 years, while the debate began in ancient Greece. Democritus and his fellows first suggested multiple kosmoi (a rough analogue to solar systems) beyond our own in the 5th century B.C. English scientist William Derham first tried to empirically confirm their existence in 1715. Aleksander Wolszczan and Dale Frail finally confirmed that existence in 1992. That’s more than 20 centuries from hypothesis to first testing, and nearly three from first empirical testing to confirmation. And although we have finally confirmed the existence of exoplanets, and although we possess methods of observation and analysis that our forebears could scarcely dream of, we should remember that this was true for them, too. Galileo’s tools would have amazed Democritus, Lowell’s would have amazed Galileo, and so on with Coblentz and Wolszczan and the scientists who have designed TESS, the Transiting Exoplanet Survey Satellite.

Despite the vast differences in era and technology, TESS and Galileo’s first telescope give us much the same things: an unorganized mass of new data drawn from the cutting edge of science’s observational and analytical capacity. But working on the cutting edge means that errors are easy to make and hard to detect. Sorting the signal from the noise—indeed, figuring out what constitutes signal in the first place—is a long and painstaking process. We don’t know if TESS’s data will contain evidence of extraterrestrial life, but we do know it’ll take a long time to figure it out. And this will be true of every new observational tool we launch after her. We’ve been discovering extraterrestrial life for 400 years, and we could easily keep doing it for 400 more.

It’s likely that a genuine discovery, if it happens, will look much like our earlier false starts. The difference is that consensus will build gradually toward confirmation rather than toward rejection. Slowly, painfully, through the weight of evidence and simple generational attrition, most practicing scientists and observers will come to agree that observed phenomena do compellingly suggest extraterrestrial life, and textbooks will be written, and truth will be made. This is how it happened for the Copernican worldview, for the plurality of worlds, for deep time and deep history, natural selection and extrasolar planets, and pretty much every other big scientific truth we have. And eventually children will learn how some brilliant scientist saw something in her instruments and knew it must be extraterrestrial life, just as Isaac Newton became the first person ever to notice gravity after an apple bopped him on the head. But we who live through it don’t get to compress the work of generations into a sentence of myth. No, we live through the whole extended, incremental, contested process. And by the time we’re sure what we’ve got, the cutting edge has already moved on.